Abstract
This paper presents a new method for sensing and the quantification of the number of solid microparticles using surface acoustic wave (SAW) devices. In contrast to the standard mass loaded delay line approach, microcavities with varying geometrical shapes and sizes are formed between SAW interdigitated transducer pairs. The system operation relies on the resonance condition occurring inside the microcavity through the coupling of Rayleigh waves to the sample, and the output phase angle is used for obtaining measurement results. It is shown through measurements that it is possible to interact with polystyrene solid microbeads trapped inside the microcavity and extract information about the size of the sample. Furthermore, the number of microbeads placed in a single file along the microcavity width can be quantified using this platform. Experimental results are compared and verified with finite-element method simulations. In essence, this novel approach resulted in a platform capable of analyzing sample volumes less than 10 pL in a non-invasive manner. For size differentiation, experimental phase shifts of 0.14° ± 0.05°, 0.81° ± 0.26°, and 3.54° ± 0.49° were obtained in rectangular microcavities for 10, 15, and 20 \mu \text{m} microbeads, respectively. On the other hand, a distribution of phase shifts as 0.51° ± 0.19°, 0.98° ±0.12°, and 1.34° ± 0.15° are obtained for counting one, two, or three microbeads, respectively. The proposed system was designed, simulated, fabricated, and tested successfully.
